Oils in contact with relatively small quantities of a contaminant liquid such as water will dissolve and absorb the liquid up to its saturation limit in the oil. An excess of the contaminant liquid beyond saturation will result in it forming a separate liquid phase within the oil. When the liquid is water, the term free water is used to describe this second liquid phase.
Oil in contact with gases (including water vapour) dissolve these gases generally in accordance with Henry's Law.
Both dissolved liquids and gases can cause problems with oils and with equipment in contact with the oils.
The main contaminant in lubricant and seal oils is water. However, hydrogen sulphide, oxygen, hydrocarbons, and other organic compounds such as alcohols, aldehydes and ketones can be dissolved and absorbed by the oils and can also form separate phases within these oils.
There are several mechanisms by which contaminants adversely effect lubrication oils. For example, when the compounds listed above are absorbed by oil, the oil viscosity is reduced and adversely affected and this affects the ability of the oil to lubricate the moving or bearing surfaces in machinery. The modification to oil viscosity normally leads to a reduction in the thickness of the protective lubricating oil film on the machinery surfaces and metal to metal contact is increased. This leads to high rates of wear and poor machinery performance.
In addition to viscosity effects, water and acid gases such as hydrogen sulphide and hydrogen cyanide cause corrosion to the surfaces they contact. Particles of corrosion products flake off metal surfaces and increase wear via abrasion of the metal surfaces.
Water and volatile gases can also cause erosion of metal surfaces via another mechanism. This erosion is caused on the metal surfaces by the rapid vaporisation that can occur as the lubricating oil containing the volatile gases heats up as it passes through and between the bearings, gears and other highly stressed surfaces causing sudden vaporisation. The resultant rapid increase in oil and gas velocity past the surfaces causes erosion. This is often referred to as cavitation.
Transformer oils are mostly contaminated by water which usually enters in the form of a gas and is absorbed into the oil. The absorbed water reduces the dielectric constant of the oil which leads to inefficiencies within the transformer and in the extreme can lead to an explosion due to arcing and vaporization of the transformer fluids.
Hydraulic oils are mostly contaminated by water which also enters as water vapour normally into the storage compartment. The dissolved water usually causes corrosion within the hydraulic system.
Edible oils, which are normally vegetable oils, contain dissolved water. The water enters the oil during the extraction process from the plant and during oil storage where water vapour condenses from air into the oil. The oil, dissolved water and free water all contain dissolved oxygen. The water in the oil allows the oxygen to act on the oil and cause oxidation and therefore rancidity of the oil, spoiling it as a foodstuff. For this reason, antioxidants are usually added to edible oils. These antioxidants are chemicals which tend to block the oxidation action of oxygen and/or water on oxidizable fractions of the oil. Without these antioxidants, edible oils would rapidly spoil and become unfit for human consumption.
Water is the principal contaminant to be removed from oils to overcome the problems described above. Water can be present in various combinations of the following forms:
Free water which is present as a separate phase from the oil and which separates as such on standing.
Emulsified water which, although present as a separate phase, is so finely dispersed that surface tension forces are not large enough to allow free settling of the water on standing. In general, emulsified water cannot be separated by purely mechanical means.
Dissolved water which is present as a solution within the oil. It is an integral part of the oil phase and cannot be remove by mechanical means (i.e. standing, filtration or centrifuging). Dissolved water exists up to the saturation limit which varies with the type of oil and its temperature. Once the saturation limit is reached, the oil cannot accommodate any more dissolved water and any excess water appears as a separate phase as either free and/or emulsified water.
In addition to water resulting from absorption into the oil from the gaseous phase, oils may be contaminated by liquid water leaking into the oil system, particularly in hydraulic and lubrication oil systems where those systems are normally cooled against cooling water. Water can also enter these systems where it condenses out of the atmosphere above the oil, especially where the oil storage reservoirs are situated in the close proximity of steam turbines or steam vents. These means of gross contamination require extensive water removal if catastrophic failure of the lubrication system and the machinery it is protecting are to be avoided.
Contamination levels of water can vary from a few hundred parts per million through to many thousands of parts per million and some lubrication systems can have periodic gross contamination of up to 10% water in the oil.
The desired level of water in the oil is less than the saturation level for that temperature. For example, most lubrication oils operate in the temperature range 30.degree. C. to 80.degree. C. At 30.degree. C., a typical saturation water level in oil is 100 ppm whereas a typical saturation water level at 80.degree. C. is 500 ppm. However, most lubrication oils give superior performance if water levels of less than 100 parts per million are present in the oil supply to the bearing or gear. A figure of less than 50 ppm in the oil supply would ensure that the oil is in a condition where it has no free water in it and will have the capacitance to absorb any liquid water or any water vapour that comes into contact with the oil. At these low levels, water is not readily available to cause viscosity changes in the oil or to cause corrosion or erosion damage.